A US fighter plane in Afghanistan has accidentally bombed a group of Canadian troops, killing four soldiers and injuring eight others.

The incident, which occurred during a routine training exercise, happened near the southern city of Kandahar, 14 kilometres (eight miles) from the airport.

US officials said initial reports suggested that the pilot of at least one of a group of F-16 fighter planes had mistaken a live-fire exercise for hostile fire.

Clearly there are many questions that the families, and all Canadians, expect to have answered

Canadian PM Jean Chretien

The aircraft reported being fired on and were given clearance by their air controller to return fire, the officials said.

The soldiers are the first Canadians to be killed in the US-led campaign against Taleban and al-Qaeda fighters in Afghanistan.

Two of those injured have life-threatening wounds and correspondents say it is the worst accidental bombing of the six-month Afghan campaign.

Canadian officials said their troops were engaged in what they described as live-fire training exercise, when a US F-16 dropped one or two 500-pound (225 kg) bombs on them.

Canada's defence chief Lieutenant General Ray Henault said the area was recognised as a training area and the aircraft were using strictly controlled routes.

Questions

The Canadian Prime Minister, Jean Chretien, said US President George Bush had called to offer condolences and pledged to co-operate with the Canadian investigation into the incident.

"As to the circumstances of what appears to have been a terrible accident, clearly there are many questions that the families, and all Canadians, expect to have answered," he said in a statement.

Reporting from the British Royal Marine's base in Bagram, the BBC's Jonathan Charles says one question which will be asked is whether the American Air National Guard should be operating in Afghanistan.

He says the pilots are reservists who generally fly civilian planes and are called up for service when required.

British marines are conducting operations in the mountains of the south-east of the country and are being given US air support for their missions - the latest "friendly fire" incident is likely to raise new safety concerns among them.

These are the first Canadian deaths in a military campaign since the Korean War in the early 1950s.

More than 700 Canadian soldiers are deployed in Afghanistan and are operating out of Kandahar.

On Monday, four American soldiers were killed while blowing up unexploded rockets, also near Kandahar.

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The BBC's David Chazan
"This was not the first time allied soldiers have been killed by their own side during the campaign"